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The powerful thrust of ocean-spawned winds can zip a kite surfer across the sea's surface at up to 55 miles per hour. Engineers are now trying to harvest the power in that wind to generate electricity. The Wing 7 airborne wind turbine pictured here is a prototype of a leading contender for the job. The autonomous, lightweight device is tethered to land or to a floating platform; when wind speeds pick up, four rotors fly it up above 820 feet in a circle perpendicular to the wind. As the air rushes across the carbon-fiber wing, the rotors generate electricity by spinning permanent magnets. “The rotors are both propellers and turbines,” notes mechanical engineer Corwin Hardham, CEO and co-founder of Makani Power, which created the Wing 7. An onboard computer makes constant adjustments.

The idea of generating power with kites, to avoid the fickleness of winds closer to the earth, is centuries old; modern iterations—including schemes for harvesting energy by flying into jet streams—date back to at least the 1970s. Makani intends to meet that challenge by designing its kite to work over the ocean, where the wind blows fairly constantly, and to cover a wide expanse of sky in its circular flight. Even relatively light winds that fail to stir traditional turbines can speed the light, tethered aircraft at more than 100 miles per hour and allow it to generate power.